The principle of spherical electromagnetic lenses having a gradient of decreasing refractive index was introduced in 1964 by Rudolf Luneburg in Mathematical Theory of Optics, Cambridge University Press. The dielectric constant of the lens, now known as Luneburg lens, is such that ∈r=2−(r/R)2,
where ∈r is the relative dielectric constant, r the position considered along the radius, and R is the radius of the lens. Obviously in this case, the dielectric permittivity shall vary from 1 to 2.
With such a lens an incoming front wave is focused at the edge of the lens, on a point opposite to the normal of the incident front wave. Using this property in the opposite direction, illuminating the lens along its edge with a selected one of several thin beams generates an emerging front wave and the selection of a particular thin beam thus allows a good control of the azimuth radiation.
Two techniques have been used for the realization of the lens: drilling, as described for instance in the article “A Sliced Spherical Lüneburg Lens”, by S. Rondineau, M. Himdi, J. Sorieux, in IEEE Antennas Wireless Propagat. Lett., 2 (2003), 163-166, or use of variable dielectric materials, such as concentric shells, as described in patent application WO 2007/003653.
These prior art solutions with spherical shapes are however not adapted to cases where the angle of radiation in elevation and the capability of beam steering in azimuth are sought.
Solutions exist that use cylindrical lenses. Such lenses are easy to feed with a known 2D electronic circuit. The width of the output beam from such lenses in the elevation plane depends on the cylindrical lenses aperture.
In particular, an antenna system comprising such a cylindrical lens has generally a gain G equal to
      G    =          32000                        θ          A                ⁢                  θ          E                      ,where θE and θA are respectively the beam width angle in elevation and azimuthal planes.
Depending on the aperture of the cylindrical lens, such an antenna system can allow either a mid-range with wide angle communications or a long-range with narrow angle communications.
However, this may not be adapted to a situation wherein different devices to reach are located at both mid-range and long-range distances from the antenna system.
Consequently, there is a need to provide an improved solution allowing performing both types of communications with the same antenna system, thus ensuring a good flexibility of transmission distance.